The invention relates to a device for detecting the speed of rotation and/or an angle of rotation of a shaft having a motion transmitter which rotates with the shaft and which has on its periphery areas of high magnetic permeability and areas of low magnetic permeability, especially a ferromagnetic motion transmitter constructed as a toothed wheel or as a perforated or slotted disc, consisting of a magnetic-field-dependent sensor, which senses the motion transmitter and is arranged at a distance from the latter, of a thin, soft-magnetic strip core, which preferably has high permeability and low hysteresis and eddy-current losses, comprising a coil which is wound onto this core transversely to the longitudinal direction and has coil connections, a permanent magnet which is likewise arranged at a distance from the motion transmitter and sensor, the sensor and permanent magnet being arranged spatially behind each other, related to the motion transmitter, and in such a manner that the lines of force of the permanent magnet pass perpendicularly through the flat side of the strip core, and of an evaluating circuit into which the coil is integrated via the coil connections.
The magnetic sensing of rotational speeds, reference markets or similar of rotating parts with the assistance of transmitters, for example by means of inductive transmitters (German Offenlegungsschrift No. 2,939,643, (German Offenlegungsschrift No. 2,924,590), Hall transmitters (German Offenlegungsschrift No. 3,231,391, German Offenlegungsschrift No. 2,814,606), magneto-resistive sensors (German Offenlegungsschrift No. 3,122,376) or Wiegand elements is known. For this purpose, all devices require as the motion transmitter ferro-magnetic transmitter wheel, mostly constructed-in the form of a toothed wheel or of a perforated or slotted disc. In this arrangement, the necessary premagnetization in the sensor is produced by at least one permanent magnet. As an alternative, the magnets can also be mounted on the motion transmitter. However, it is always the modulation of the magnetic flux by the sensor as a consequence of the rotational movement which is utilized for detecting the signal.
In the automotive field, the inductive transmitter has heretofore been most successful since it is rugged and of simple construction and has a large range of operating temperatures. However, the detection of very low rotation speeds such as occur, for example, with anti-blocking systems or with the forward drive control system, its use is associated with problems since its output signal is a function of the rotational speed.
This situation is not remedied by the generic device according to German Offenlegungsschrift No. 3,205,625 which uses an inductive transmitter comprising two sensor coils and in which the quotient of the two coil inductances is evaluated since this only eliminates possible air-gap fluctuations between the sensor and the motion transmitter.